Hot-rolled steel sheet for high strength linepipe having tensile strength of 540 mpa or more

ABSTRACT

A hot-rolled steel sheet for a high strength linepipe having a tensile strength of 540 MPa or more and a method for making the same. The steel sheet having a chemical composition containing, by mass %, C: 0.02% or more and 0.06% or less, Si: 0.05% or more and 0.25% or less, Mn: 0.60% or more and 1.10% or less, P: 0.008% or less, S: 0.0010% or less, Nb: 0.020% or more and 0.060% or less, Ti: 0.001% or more and 0.020% or less, Al: 0.01% or more and 0.08% or less, Ca: 0.0005% or more and 0.0050% or less, one or more selected from among Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, and V: 0.10% or less, and the balance being Fe and inevitable impurities, in which the relationships 0.60≦CP≦0.90 and CM≦3.05 are satisfied.

TECHNICAL FIELD

This application is directed to a hot-rolled steel sheet having hydrogen induced cracking resistance (hereinafter, called HIC resistance) and a strength of ×70 or more in accordance with API (American Petroleum Institute) standards which can suitably be used as a material for an electric resistance welded steel pipe for a linepipe for transporting energy resources such as crude oil and a natural gas and to a method for manufacturing the steel sheet.

BACKGROUND

UOE steel pipes have been mainly used for linepipes to date from the viewpoint of transport efficiency, because steel pipes having a large diameter and a large thickness can be manufactured using a UOE steel pipe. However, high strength electric resistance welded steel pipes, which are manufactured from hot-rolled steel sheets in a coil shape (hot-rolled steel strips) that are less expensive and have high productivity as a material, are being increasingly used for linepipes instead of UOE steel pipes nowadays. Electric resistance welded steel pipes have an advantage in that they are superior to UOE steel pipes in terms of deviation of a wall thickness and roundness in addition to cost advantage. On the other hand, since the pipe production method for electric resistance welded steel pipes involves cold roll forming, the method is characteristic of much more plastic strain being given to steel pipes than to a HOE steel pipe when pipe production is performed.

Nowadays, regarding exploitation of crude oil and a natural gas, there is a growing tendency for oil wells and gas wells to be developed in the polar areas or in deep regions due to an increase in the demand for energy and due to the progress of mining technology. Linepipes which are used at such places are required to have so-called sour resistance such as HIC resistance and sulfate stress corrosion cracking resistance (SSC resistance) in addition to strength, toughness, and weldability. In the case of linepipes, which are not given stress after having been laid, HIC resistance is particularly important.

HIC is a phenomenon in which hydrogen ions having been generated by a corrosion reaction become hydrogen atoms on the surface of a steel, and the hydrogen atoms enter into the steel, accumulate around inclusions such as MnS, carbides having a large grain diameter such as NbC, and a second hard phase so as to increase internal pressure and cause the steel material to eventually crack. In addition, in the case where a steel material is given plastic strain, many dislocations are formed around the inclusions, carbides, and the second hard phase mentioned above, and hydrogen atoms are more likely to accumulate, which results in HIC being more likely to occur.

To date, various solutions have been proposed in order to solve the problem of HIC described above.

Patent Literature 1 discloses a method for improving HIC resistance in which inclusions, which become the origins of HIC, are rendered harmless by controlling the total contents of chemical elements which combine respectively with S, O (oxygen), and N to form inclusions to be 0.01% or less or by controlling the maximum diameter of inclusions to be 5 μm or less, and in which the hardness of a center segregation part is controlled to be Hv 330 or less.

Patent Literature 2 discloses a method for decreasing the area ratio of HIC by decreasing the size of TiN grains, which become the origin of HIC. Specifically, the size of Al—Ca-based sulfides in molten steel is decreased by controlling a weight ratio CaO/Al₂O₃ to be 1.2 to 1.5 by adjusting the added contents of Al and Ca, and the grain diameter of Al—Ti—Ca-based complex inclusions which are formed using the sulfides as nuclei is controlled to be 30 μm or less.

In addition, Patent Literature 3 discloses a method in which the formation of carbonitrides of Nb and Ti, which become the origins of HIC, is less likely to occur by controlling Nb concentration to be 0.060% or less and Ti concentration to be 0.025% or less in a region located at a distance in the thickness direction of 5% of the thickness from the central part in the thickness direction.

Patent Literature 4 discloses a method for manufacturing a high strength linepipe excellent in terms of HIC resistance in which HIC resistance is improved by decreasing the degree of center segregation as a result of decreasing Mn content added in steel and in which Cr and Mo, which are comparatively less likely to undergo center segregation, are utilized.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2006-63351

PTL 2: Japanese Patent No. 4363403 (International Publication No. WO2005/075694)

PTL 3: Japanese Unexamined Patent Application Publication No. 2011-63840

PTL 4: Japanese Patent No. 2647302 (Japanese Unexamined Patent Application Publication No. 5-271766)

SUMMARY Technical Problem

Although it is possible to render the origins of HIC harmless to some extent using the methods disclosed in Patent Literature 1 to Patent Literature 3, there is an increase in sensitivity for HIC in the case of a high strength steel sheet of ×70 or more in accordance with API standards in particular, and therefore a sufficient effect is not realized only by controlling the amount and size of inclusions.

In addition, in the case where the method disclosed in Patent Literature 4, in which Cr and Mo are utilized, is used, if Cr and Mo are excessively added, there is an increased tendency for the formation of a martensite phase to occur in a center segregation part, and therefore there is a problem of a deterioration in HIC resistance.

Disclosed embodiments have been completed in view of the situation described above, and an object of disclosed embodiments is to provide a hot-rolled steel sheet for a high strength linepipe excellent in terms of HIC resistance which can suitably be used as a raw material of a high strength electric resistance welded linepipe of ×70 or more in accordance with API standards.

Here, “excellent in terms of HIC resistance” refers to a case where a crack length ratio (CLR) is 15% or less after a steel sheet has been immersed in a NACE solution (NACE TM-0284 solution A: 5% Nacl+0.5% CH₃OOH, 1 atmosphere, saturated with H₂S, and pH=3.0 to 4.0) for 96 hours.

Solution to Problem

Disclosed embodiments have been completed on the idea that, in the case of a hot-rolled steel sheet for a high strength linepipe having a TS of 540 MPa or more where there is an increase in sensitivity for HIC, a crack length ratio CLR is controlled to be small even in the presence of some amount of inclusions which become the origins of the occurrence of HIC, by improving propagation resistance of HIC as a result of decreasing the grain diameter of a microstructure in a center segregation part through an improvement in the hardenability of the center segregation part by controlling the chemical composition of steel. That is to say, the subject matter of disclosed embodiments is as follows.

[1] A hot-rolled steel sheet for a high strength linepipe having a tensile strength of 540 MPa or more and excellent HIC resistance, the steel sheet having a chemical composition containing, by mass %, C: 0.02% or more and 0.06% or less, Si: 0.05% or more and 0.25% or less, Mn: 0.60% or more and 1.10% or less, P: 0.008% or less, S: 0.0010% or less, Nb: 0.020% or more and 0.060% or less, Ti: 0.001% or more and 0.020% or less, Al: 0.01% or more and 0.08% or less, Ca: 0.0005% or more and 0.0050% or less, one or more selected from among Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.05% or less, and V: 0.10% or less, and the balance being Fe and inevitable impurities, in which the relational expression (1) below is satisfied.

0.60≦CP≦0.90   (1),

where CP is calculated from CP=4.46×C+2.37×Mn/6+(1.18×Cr+1.95×Mo+1.74×V)/5+(1.74×Cu+1.70×Ni)/15, where the atomic symbols in the equation represent respectively the contents of the corresponding chemical elements by mass %.

[2] The hot-rolled steel sheet for a high strength linepipe having a tensile strength of 540 MPa or more and excellent HIC resistance according to item [1], in which relational expression (2) below is satisfied in addition to having the chemical composition.

CM≦3.05   (2),

where CM is calculated from CM=2.37×Mn2.34×Mo+0.59×Cr+0.17×Ni, where the atomic symbols in the equation represent respectively the contents of the corresponding chemical elements by mass %.

[3] The hot-rolled steel sheet for a high strength linepipe having a tensile strength of 540 MPa or more and excellent HIC resistance according to item [1] or [2], in which the steel sheet has a metallographic structure including, in terms of area fraction, 95% or more of a bainitic-ferrite microstructure in a center segregation part in addition to having the chemical composition, and the average grain diameter of the bainitic-ferrite microstructure is 8.0 μm or less.

A method for manufacturing a hot-rolled steel sheet for a high strength linepipe having a tensile strength of 540 MPa or more and excellent HIC resistance, the method including heating a steel slab having the chemical composition according to item [1] or [2] at a temperature of 1100° C. or higher and 1300° C. or lower, performing rough rolling on the steel slab, thereafter performing finish rolling on the rough-rolled steel under condition that cumulative rolling reduction ratio is 20% or more in a temperature range of 930° C. or lower, performing accelerated cooling on the finish-rolled steel sheet to a temperature of 380° C. or higher and 600° C. or lower at an average cooling rate of 10° C./s or more and 100° C./s or less in terms of the temperature of the central part in the thickness direction, and coiling the cooled steel sheet into a coil shape.

Advantageous Effects

According to embodiments, even in the presence of some amount of inclusions, it is possible to suppress HIC by controlling to refine a microstructure in a center segregation part to be small and to manufacture a high strength hot-rolled steel sheet excellent in terms of HIC resistance which can suitably be used for an electric resistance welded steel pipe for a linepipe of ×70 or more in accordance with API standards which can be used without causing any problem even in a harsh environment equivalent to a NACE solution. In addition, the hot-rolled steel sheet manufactured according to embodiments can also be used for a spiral steel pipe for a linepipe of ×70 or more in accordance with API standards.

DETAILED DESCRIPTION

The reasons for the features of disclosed embodiments will be described hereafter.

1. Regarding Chemical Composition

First, the reasons for the limitations on the chemical composition of the steel according to embodiments will be described. Here, % used when describing the chemical composition always represents mass %.

C: 0.02% or more and 0.06% or less

C is a chemical element which significantly contributes to an increase in the strength of steel, and such an effect is realized in the case where the C content is 0.02% or more, but, in the case where the C content is more than 0.06%, since a second phase such as a pearlite microstructure is easy to be formed, there is a deterioration in HIC resistance. Therefore, the C content is set to be 0.02% or more and 0.06% or less, or preferably 0.03% or more and 0.05% or less.

Si: 0.05% or more and 0.25% or less

Si is a chemical element which is added for solute strengthening and decreasing scale-off quantity when hot rolling is performed, and such an effect is realized in the case where the Si content is 0.05% or more, but, in the case where the Si content is more than 0.25%, since red scale excessively grows, cooling ununiformity occurs when hot rolling is performed, which results in a deterioration in the appearance and the uniformity of material properties. Therefore, the Si content is set to be 0.05% or more and 0.25% or less, or preferably 0.10% or more and 0.25% or less. In addition, since there is a deterioration in toughness in an electric resistance weld zone as a result of forming MnSi-based oxides when electric resistance welding is performed, it is preferable that Si be added so that the ratio Mn/Si is 4.0 or more and 12 or less.

Mn: 0.60% or more and 1.10% or less

Mn is a chemical element which contributes to an improvement in strength and toughness as a result of refining of a steel microstructure, and such an effect is realized in the case where the Mn content is 0.60% or more. On the other hand, in the case where the Mn content is increased, since a fine martensite microstructure is more likely to be formed in a center segregation part, and since MnS, which becomes the origin of HIC, is more likely to be formed, it is necessary that the Mn content be controlled to be 1.10% or less. Therefore, the Mn content is set to be 0.60% or more and 1.10% or less, preferably 0.80% or more and 1.10% or less, or more preferably 0.80% or more and 1.05% or less.

P: 0.008% or less

Since P is a chemical element which is contained as an inevitable impurity, and since P deteriorates HIC resistance as a result of significantly increasing the hardness of a center segregation part, it is preferable that the P content be as small as possible, but a P content of 0.008% or less is acceptable. Moreover, since there is an increase in cost due to an increase in refining time in order to markedly decrease the P content, it is preferable that the P content be 0.002% or more.

S: 0.0010% or less

Since S is, like P, a chemical element which is inevitably contained in steel, and since S forms MnS in steel, it is preferable that the S content be as small as possible, but a S content of 0.0010% or less is acceptable. The S content is preferably 0.0006% or less.

Nb: 0.020% or more and 0.060% or less

Nb is a chemical element which contributes to an increase in the strength of steel as a result of precipitating in the form of fine Nb carbonitrides in a coiling process when hot rolled steel sheets are manufactured. Also, Nb is a chemical element which contributes to an improvement in the toughness of a weld zone as a result of suppressing the growth of austenite grains when electric resistance welding is performed. Such effects are realized in the case where the Nb content is 0.020% or more. On the other hand, in the case where the Nb content is more than 0.060%, Nb carbonitrides having a large grain diameter, which become the origins of HIC, are more likely to be formed. Therefore, the Nb content is set to be 0.020% or more and 0.060% or less, or preferably 0.030% or more and 0.050% or less.

Ti: 0.001% or more and 0.020% or less

Ti is a chemical element which is added in order to render N, which significantly deteriorates the toughness of steel, harmless by fixing N in the form of TiN. Such an effect is realized in the case where the Ti content is more than 0.001%. On the other hand, in the case where the Ti content is more than 0.020%, since there is an increase in the amount of Ti carbonitrides which precipitate along the cleavage plane of Fe, there is a deterioration in the toughness of steel. Therefore, the Ti content is set to be 0.001% or more and 0.020% or less, or preferably 0.005% or more and 0.015% or less.

Al: 0.01% or more and 0.08% or less

Although Al is added as a deoxidation agent, in the case where the Al content is more than 0.08%, there is insufficient deoxidation effect in the case where the Al content is less than 0.01%, and, on the other hand, there is a deterioration in HIC resistance and toughness due to an increase in the amount of coarse Al-based oxides remaining in steel. Therefore, the Al content is set to be 0.01% or more and 0.08% or less, or preferably 0.01% or more and 0.05% or less.

Ca: 0.0005% or more and 0.0050% or less

Ca is a chemical element which is effective for improving HIC resistance by shape control of sulfide-based inclusions, and such an effect is realized in the case where the Ca content is 0.0005% or more. On the other hand, in the case where the Ca content is more than 0.0050%, such an effect becomes saturated, and, in addition, there is a deterioration in HIC resistance as a result of generation of a large amount of Ca oxides. Therefore, the Ca content is set to be 0.0005% or more and 0.0050% or less, or preferably 0.0010% or more and 0.0030% or less.

According to embodiments, one or more selected from among Cu, Ni, Cr, Mo, and V may be further added in the amounts described below.

Cu: 0.50% or less

Cu is a chemical element which contributes to an improvement in the toughness and strength of steel through an improvement in hardenability, and, since Cu is less likely to be concentrated in a center segregation part than Mn and Mo which have similar effect as Cu, Cu can increase the strength of steel without decreasing HIC resistance. Therefore, Cu is added in accordance with the strength grade of steel. Such an effect is realized in the case where the Cu content is 0.05% or more, but, in the case where the Cu content is more than 0.50%, the effect becomes saturated and there is an unnecessary increase in cost in such case. Therefore, the Cu content is 0.50% or less, or preferably 0.40% or less.

Ni: 0.50% or less

Ni is, like Cu, a chemical element which contributes to an improvement in the toughness and strength of steel through an improvement in hardenability, and, since Ni is less likely to be concentrated in a center segregation part than Mn and Mo which have a similar effect, Ni can increase the strength of steel without deteriorating HIC resistance. Therefore, Ni is added in accordance with the strength grade. Such an effect is realized in the case where the Ni content is 0.05% or more, but, in the case where the Ni content is more than 0.50%, the effect becomes saturated and there is an unnecessary increase in cost in such case. Therefore, the Ni content is 0.50% or less, or preferably 0.40% or less.

Cr: 0.50% or less

Cr is a chemical element which is effective for improving the toughness and strength of steel by improving hardenability, and such an effect is realized in the case where the Cr content is 0.05% or more. However, Cr significantly deteriorates the toughness of a weld zone as a result of forming Cr oxides when electric resistance welding is performed. In order to suppress such a deterioration, the Cr content is set to be 0.50% or less, or preferably 0.30% or less.

Mo: 0.50% or less

Mo is a chemical element which is very effective for improving the toughness and strength of steel by improving hardenability, and such an effect is realized in the case where the Mo content is 0.05% or more, but, in the case where the Mo content is more than 0.50%, the effect becomes saturated and there is an unnecessary increase in cost in such case. Therefore, the Mo content is set to be 0.50% or less, or preferably 0.30% or less.

V: 0.10% or less

V is a chemical element which contributes to an increase in the strength of steel through solute strengthening and precipitation strengthening in the case where the V content is 0.005% or more, but, in the case where the V content is more than 0.10%, since there is an increase in the hardness of a center segregation part, there is a deterioration in HIC resistance. Therefore, the V content is set to be 0.10% or less, or preferably 0.080% or less.

CP: 0.60 or more and 0.90 or less

According to embodiments, a CP value, which is determined by the contents of various alloy chemical elements, satisfies relational expression (1) below.

0.60≦CP≦0.90   (1),

where CP is calculated from CP=4.46×C+2.37×Mn/6+(1.18×Cr+1.95×Mo+1.74×V)/5+(1.74×Cu+1.70×Ni)/15, where the atomic symbols in the equation represent respectively the contents of the corresponding chemical elements by mass %, where the atomic symbol corresponding to a chemical element which is not added being assigned a value of 0.

The CP value is an index indicating the hardenability of a center segregation part. It is possible to obtain a fine bainitic-ferrite microstructure having a grain diameter of 8.0 μm or less in the center segregation part by controlling the chemical composition of steel so that the CP value is 0.60 or more. On the other hand, in the case where the CP value is more than 0.90, there is an excessive improvement in hardenability, and therefore there is an increase in the hardness of a center segregation part. Accordingly, the CP value is set to be 0.60 or more and 0.90 or less, or preferably 0.70 or more and 0.90 or less.

CM: 3.05 or less

A fine martensite microstructure which is formed in a center segregation part deteriorates HIC resistance. The chemical elements which contribute to the formation of a fine martensite microstructure are Mn, Mo, Cr, and Ni, and the degree of influence of these chemical elements on the amount of a fine martensite microstructure formed is numerically represented by a CM value. It is necessary that the value of CM shown below satisfy relational expression (2) below in order to control the area fraction of a fine martensite microstructure which is formed in a center segregation part to be less than 5%.

CM≦3.05   (2),

where CM is calculated from CM=2.37×Mn+2.34×Mo+0.59×Cr+0.17×Ni, where the atomic symbols in the equation represent respectively the contents of the corresponding chemical elements by mass %. It is preferable that the value of CM be 2.95 or less.

Here, the remainder of the chemical composition other than constituents described above is Fe and inevitable impurities. However, other small amounts of elements may be added as long as the effects of disclosed embodiments are not decreased.

2. Regarding Metallographic Structure

Subsequently, the metallographic structure according to disclosed embodiments will be described.

It is necessary that the metallographic structure of the disclosed embodiments be a bainitic-ferrite microstructure having excellent toughness. In the case where other kinds of microstructures such as a fine martensite microstructure, an upper bainite microstructure, and a pearlite microstructure are present in a bainitic-ferrite microstructure, since these other kinds of microstructures become hydrogen trapping sites, there is a deterioration in HIC resistance. Therefore, it is preferable that the fractions of the microstructures other than a bainitic-ferrite microstructure be as small as possible. However, in the case where the area fractions of the microstructures other than a bainitic-ferrite microstructure are markedly small, since the influences of the microstructures other than a bainitic-ferrite microstructure are negligibly small, the microstructures other than a bainitic-ferrite microstructure may be included to some extent. Specifically, a case where the total area fraction of the steel microstructures (such as a fine martensite microstructure, an upper bainite microstructure, and a pearlite microstructure) other than a bainitic-ferrite microstructure in the center segregation part is 5% or less is included in embodiments.

Average grain diameter of a bainitic-ferrite microstructure: 8.0 μm or less

It is necessary that the average grain diameter of a bainitic-ferrite microstructure be 8.0 μm or less in order to achieve sufficient toughness (vTrs≦−80° C.) for a steel sheet used for a linepipe. Also, it is desirable that the average grain diameter of a bainitic-ferrite microstructure be 8.0 μm or less in order to improvement the crack propagation resistance of HIC. It is preferable that the average grain diameter of a bainitic-ferrite microstructure be 6.0 μm or less.

3. Regarding Manufacturing Conditions

Subsequently, manufacturing conditions for achieving the steel microstructure described above will be described.

A slab heating temperature is set to be 1100° C. or higher and 1300° C. or lower. In the case where the temperature is lower than 1100° C., since the temperature is not high enough for carbides, which are formed in steel when continuous casting is performed, to be solid-solute completely, the required strength is not achieved. On the other hand, in the case where the temperature is higher than 1300° C., since there is a marked coarsening of austenite grain, there is a deterioration in toughness. Here, this temperature refers to the temperature of the interior of the heating furnace, and the center of the slab is presumed to be heated to this temperature.

In the finish rolling step, it is necessary that finish rolling be performed under the condition that cumulative rolling reduction ratio is 20% or more in a temperature range of 930° C. or lower. In the case where the cumulative rolling reduction ratio is less than 20%, since there are an insufficient number of nucleation sites of a bainitic-ferrite microstructure, the microstructure becomes coarse, which results in a deterioration in toughness. However, in the case where the cumulative rolling reduction is more than 80%, since the effect becomes saturated, and since a so high load is applied to a rolling mill, it is preferable that the upper limit of cumulative rolling reduction ratio be 80% or less.

The average cooling rate of the central part in the thickness direction of a steel sheet is set to be 10° C./s or more and 100° C./s or less. In the case where the cooling rate is less than 10° C./s, the area fractions of a ferrite microstructure and/or a pearlite microstructure become more than 5% even if hardenability increasing chemical elements such as Cu, Ni, and Cr are added. Therefore, it is necessary that the cooling rate be 10° C./s or more. On the other hand, in the case where the cooling rate is more than 100° C./s, the area fraction of a martensite microstructure becomes more than 5%. The cooling rate of the central part in the thickness direction of a steel sheet was calculated by using the temperature history of the central part in the thickness direction of the steel sheet by performing heat-transfer calculation using the cooling capacity and heat-transfer coefficient of a run-out, which had been investigated in advance, and the surface temperature of the steel sheet, which had been determined using a radiation thermometer on the run-out.

The cooling stop temperature is set to be 380° C. or higher and 600° C. or lower. In the case where the cooling stop temperature is higher than 600° C., since there is coarsening of precipitation strengthening grains such as Nb carbonitrides, there is a decrease in strength. Moreover, since there is an enhancement of increase in the concentration of carbon in a center segregation part, a fine martensite microstructure, an upper bainite microstructure, and a pearlite microstructure tend to be formed. On the other hand, in the case where the cooling stop temperature is lower than 380° C., since there is an improvement in the deformation resistance of a steel sheet, it is difficult to coil the steel sheet into a coil shape, and there is a decrease in strength due to precipitation strengthening grains such as Nb carbonitrides not being precipitated.

EXAMPLES

By performing hot rolling on steel materials having the chemical compositions given in Table 1 under the hot rolling conditions and the cooling conditions given in Table 2, and by coiling the hot-rolled steel sheets into a coil shape, hot-rolled steel sheets having the thicknesses given in Table 2 were obtained.

TABLE 1 Steel CP CM Grade C Si Mn P S Nb Ti Al Ca Cu Ni Cr Mo V Value*1 Value*2 Note A 0.04 0.19 1.01 0.006 0.0005 0.050 0.012 0.047 0.0022 0.34 0.34 — — 0.07 0.81 2.45 Example B 0.05 0.13 1.02 0.007 0.0006 0.044 0.010 0.040 0.0025 0.46 0.46 — — — 0.89 2.50 Example C 0.05 0.13 0.85 0.005 0.0004 0.042 0.008 0.038 0.0028 0.01 0.01 0.23 0.12 0.02 0.78 2.43 Example D 0.04 0.15 1.05 0.005 0.0004 0.030 0.009 0.033 0.0030 0.17 0.16 0.13 0.18 0.06 0.85 3.01 Example E 0.03 0.10 0.74 0.005 0.0006 0.042 0.008 0.045 0.0025 — — — 0.10 0.02 0.58 1.99 Comparative Example F 0.05 0.13 1.05 0.006 0.0004 0.040 0.010 0.034 0.0021 0.30 0.30 0.15 0.25 — 0.97 3.21 Comparative Example G 0.04 0.19 1.45 0.006 0.0009 0.034 0.009 0.040 0.0025 0.01 0.01 — — 0.02 0.89 3.44 Comparative Example H 0.04 0.21 1.00 0.007 0.0005 0.005 0.008 0.045 0.0024 0.01 0.01 — 0.10 — 0.77 2.61 Comparative Example Annotation: An underlined portion indicates a value out of the range according to disclosed embodiments. *1CP = 0.46 × C + 2.37 × Mw/5 + (1.16 × Cr + 1.95 × Mo + 1.74 × V)/5 + (1.74 × Cu + 1.70 × Ni)/15, where the atonic symbols in the equation represent/respectively the contents of the corresponding chemical elements by mass %. *2CM = 2.37 × Mn + 2.34 × Mo + 0.59 × Cr + 0.17 × Ni, where the atomic symbols in the equation represent/respectively the contents of the corresponding chemical elements by mass %.

TABLE 2 Cumulative Rolling Finish Reduction Rolling Steel Slab Heating Ratio in Delivery Average Cooling Rate of Cooling Stop Sheet Steel Thickness Temperature Finish Rolling Temperature Central Part in Thickness Temperature No. Grade (mm) (° C.) (%) (° C.) Direction (° C./s) (° C.) Note 1 A 16 1200 25 840 25 450 Example 2 A 20 1200 45 840 15 500 Example 3 A 10 1200 55 810 50 520 Example 4 A 8 1150 65 810 70 420 Example 5 B 12 1200 55 810 10 520 Example 6 C 12 1200 65 810 25 560 Example 7 D 25 1200 40 820 60 420 Example 8 D 25 1200 40 820 55 480 Example 9 D 25 1200 55 810 30 530 Example 10 A 16 1350 55 810 10 510 Comparative Example 11 A 20 1200 10 830 15 500 Comparative Example 12 A 10 1250 40 820 120 520 Comparative Example 13 A 8 1150 50 830 20 650 Comparative Example 14 E 19 1150 55 810 10 510 Comparative Example 15 F 19 1200 33 830 15 500 Comparative Example 16 G 19 1250 40 820 15 520 Comparative Example 17 H 16 1150 50 830 20 500 Comparative Example

Test pieces were collected from the obtained hot-rolled steel sheets, and by performing microstructure observation, a tensile test, a Charpy impact test, hardness determination, and a HIC resistance test, tensile properties, toughness, and HIC resistance were evaluated.

By collecting a test piece for microstructure observation from the obtained hot-rolled steel sheet, by polishing a cross section of the test piece in the rolling direction, by immersing the test piece in a 2% nital solution for 30 seconds or more in order to expose segregation lines, and by then using an electron scanning microscope (at a magnification of 2000 times), photographs were taken for 5 microscopic fields or more at the segregation position in order to determine the kinds of microstructures, the grain size of a bainitic-ferrite microstructure, and the area fractions of harmful second phases such as a fine martensite microstructure, an upper bainite microstructure, and a pearlite microstructure. The steel microstructure was identified using the test piece for microstructure observation which was collected from the position located at ¼ t in the thickness direction of the steel sheet.

A tensile test piece was collected from the obtained hot-rolled steel sheet so that the longitudinal direction was at a right angle to the rolling direction (C direction), and a tensile test was performed at room temperature in accordance with API-5L specification in order to determine yield stress YS (deformation stress for a nominal strain of 0.5%) and tensile stress TS.

A V-notched test piece was collected from the central part in the thickness direction of the obtained hot-rolled steel sheet so that the longitudinal direction was at a right angle to the rolling direction (C direction), and absorbed energy and a percent brittle fracture were determined by performing Charpy impact tests at temperatures in the range of −140° C. to 0° C. in accordance with JIS Z 2242 in order to determine a temperature (fracture transition temperature) at which the percent brittle fracture was 50%. Here, three test pieces were used for one temperature in order to obtain the respective arithmetic averages of the determined absorbed energy and percent brittle fracture.

A case where the fracture transition temperature (vTrs) was −80° C. or lower was judged as satisfactory (O).

A HIC test piece having the thickness of the steel sheet, a width of 20 mm, and a length of 100 mm was collected from the obtained hot-rolled steel sheet so that the longitudinal direction was the rolling direction of the steel sheet, and a HIC resistance test was performed using an A solution in accordance with NACE TM 0284 in order to evaluate HIC resistance. Here, 10 test pieces were used for one coil, and a compressive strain of 10% was applied in the width direction to the test pieces in advance in order to simulate influence of plastic strain applied to a steel sheet in a process of forming an electric resistance welded steel pipe. From the test results, in the case where the crack length ratios (CLR) of all the test pieces for one coil were 15% or less, the coil was judged as satisfactory (O) in terms of HIC resistance. In the case where the crack length ratios of one or more of the test pieces for one coil were more than 15%, the coil was judged as unsatisfactory (x) in terms of HIC resistance.

The obtained results are given in Table 3.

TABLE 3 BF Grain Phase Metallographic Diameter Fraction structure in Center Other than Steel Motallographic of Center Segregation BF in Center Yield Tensile Sheet Steel structure of Non- Segregation Part Segregation Strength Strength Charpy No. Grade segregation Part Part (μm) Part (%) (MPa) (MPa) (vTrs) HIC Resistance Note 1 A BF BF 4.0 0.0 502 568 −80 ◯ Example 2 A BF BF 5.4 0.0 506 573 −100 ◯ Example 3 A BF BF + M 5.6 0.2 526 598 −90 ◯ Example 4 A BF BF 4.2 0.0 510 566 −130 ◯ Example 5 B BF BF + M 3.6 0.5 501 565 −100 ◯ Example 6 C BF BF 6.0 0.0 493 575 −95 ◯ Example 7 D BF BF + M 4.5 0.5 502 567 −95 ◯ Example 8 D BF BF + M 4.8 0.8 536 606 −95 ◯ Example 9 D BF BF + M 5.2 1.2 548 637 −85 ◯ Example 10 A BF BF 8.7 0.0 516 585 −45 X Comparative Example 11 A BF BF 6.6 0.0 511 575 −55 ◯ Comparative Example 12 A B + M B + M — B: 30, M: 70 648 770 −15 X Comparative Example 13 A BF + F + P F + P — F: 80, P: 20 504 600 −40 X Comparative Example 14 E BF BF 8.5 0.0 429 477 −110 X Comparative Example 15 F BF + M BF + M 3.5 7.5 507 617 −90 X Comparative Example 16 G BF BF + M 4.0 9.0 518 595 −80 X Comparative Example 17 H BF BF 7.6 0.0 379 430 −40 ◯ Comparative Example Annotation: BF: bainitic-ferrite, B: banitine, M: martensile, F: ferrite, P: pearlite An underlined portion indicates a value out of the range according to embodiments.

The examples according to embodiments are all steel sheets having a high strength of 540 MPa or more and excellent HIC resistance. On the other hand, the comparative examples, which were out of the range of disclosed embodiments, did not achieve the desired properties as a hot rolled steel sheet for high strength electric resistance welded steel pipes excellent in terms of HIC resistance, because the desired strength or toughness was not achieved, or because there was a deterioration in HIC resistance. 

1. A hot-rolled steel sheet for a high strength linepipe, the steel sheet having a tensile strength of 540 MPa or more and having a chemical composition comprising: C: 0.02% or more and 0.06% or less, by mass %; Si: 0.05% or more and 0.25% or less, by mass %; Mn: 0.60% or more and 1.10% or less, by mass %; P: 0.008% or less, S: 0.0010% or less, by mass %; Nb: 0.020% or more and 0.060% or less, by mass %; Ti: 0.001% or more and 0.020% or less, by mass %; Al: 0.01% or more and 0.08% or less, by mass %; Ca: 0.0005% or more and 0.0050% or less, by mass %; one or more selected from the group consisting of Cu: 0.50% or less, by mass %, Ni: 0.50% or less, by mass %, Cr: 0.50% or less, by mass %, Mo: 0.50% or less, by mass %, and V: 0.10% or less, by mass %; and remaining Fe and unavoidable impurities as a balance, wherein the relational expressions (1) and (2) below are satisfied: 0.60≦CP≦0.90   (1), where CP is calculated from CP=4.46×C+2.37×Mn/6+(1.18×Cr+1.95×Mo+1.74×V)/5+(1.74×Cu+1.70×Ni)/15, where the atomic symbols in the equation represent respectively the contents of the corresponding chemical elements by mass %; and CM≦3.05   (2), where CM is calculated from CM=2.37×Mn+2.34×Mo+0.59×Cr+0.17×Ni, where the atomic symbols in the equation represent respectively the contents of the corresponding chemical elements by mass %.
 2. The hot-rolled steel sheet for a high strength linepipe according to claim 1, wherein the steel sheet has a metallographic structure including a bainitic-ferrite microstructure in a center segregation part, the bainitic-ferrite microstructure being present in the center segregation part in the range of 95% or more in terms of area fraction, and the average grain diameter of the bainitic-ferrite microstructure is in the range of 8.0 μm or less.
 3. The hot-rolled steel sheet for a high strength linepipe according to claim 2, wherein the steel sheet has a yield strength in the range of 493 MPa to 548 MPa.
 4. The hot-rolled steel sheet for a high strength linepipe according to claim 2, wherein the steel sheet has a tensile strength in the range of 565 MPa to 637 MPa.
 5. The hot-rolled steel sheet for a high strength linepipe according to claim 2, wherein the steel sheet has a fracture transition temperature in the range of −80° C. or less.
 6. A method for manufacturing a hot-rolled steel sheet for a high strength linepipe, the method comprising: heating a steel slab having the chemical composition according to claim 1 at a temperature in the range of 1100° C. to 1300° C.; rough rolling on the steel slab; then finish rolling the rough-rolled steel slab to generate a finish-rolled steel sheet under conditions that a cumulative rolling reduction ratio is in the range of 20% or more in a temperature in the range of 930° C. or less; cooling the finish-rolled steel sheet to a temperature in the range of 380° C. to 600° C. at an average cooling rate in the range of 10° C./s to 100° C./s in terms of a temperature of a central part of the finish-rolled steel sheet in a thickness direction; and coiling the cooled finish-rolled steel sheet into a coil shape, wherein the cooled finish-rolled steel sheet has a tensile strength of 540 MPa or more. 